1. Field
The invention relates to the field of measuring biometric signals.
2. Description of the Related Art
When measuring biometric signals, skin electrodes are typically in contact with the skin of a subject whose biometric signals are being measured. The skin electrodes sense electric signals, such as electrocardiographic or electromyographic signals, from the human body, and it may be understood that the contact between the skin and a skin electrode forms an impedance circuit. Particularly, when carrying out measurements on a moving body, a skin electrode may be in continuous motion with respect to the skin, and the motion causes variance in properties of the impedance circuit. It may be understood that the motion modulates the impedance of the impedance circuit, thereby causing motion artefacts superposed to the biometric signal. Typically, skin electrodes located in different parts of the body move differently with respect to the body and, accordingly, the motion artefacts may differ between two (or more) skin electrodes. This interferes with the measurement of the biometric signals.
Furthermore, human skin contains electric charges that are typically distributed non-uniformly. Additionally, the contact between the skin electrodes and the subject's skin causes a triboelectric effect, also known as triboelectric charging. The triboelectric effect is a type of contact electrification in which materials in contact with each other become electrically charged as a result of their movement with respect to each other (such as through rubbing). The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties. Therefore, the triboelectric effect is different for different skin electrodes in contact with the subject. As a result of these phenomena, for example, electrical charges around one skin electrode are typically different from electrical charges around another skin electrode, which causes interference to the measurement of biometric signals.